Using my nifty thermal camera, I discovered that my furnace closet's combustion air duct (visible in this question) is getting really hot when the attic heats up, and then (being sheet metal) it radiates that heat into the furnace closet. Here's the combustion air duct:
The furnace closet has no insulation to thermally separate it from the rest of the house, so the obvious thing happens: the heat escapes. Here's a thermal view of the wall closest to the duct when it's hot out:
In order to combat this heat transfer, I would like to insulate the stud walls that frame the furnace closet, which are currently empty. The furnace closet sits above a return air plenum that is un-drywalled, giving me a place where I could stuff something rigid or semi-rigid up into the cavities from the bottom. I am considering using Roxul mineral wool insulation batts, which I hear are more rigid and dimensionally stable than fiberglass batts. Would this work? are they rigid enough that I could actually stuff them up into the cavity? Or would this be a waste of time and money because they'll just snag and compress?
In addition, would a radiant barrier help? (asked as a separate question)
Before anyone asks, I don't want to fill the stud bays with loose fill cellulose because it would require drywalling and sealing the open sections in the return air plenum and blowing the material in from the attic, which is scary and I don't like to go up there. It's also flammable and it would be used to insulate a furnace closet, after all.